1. Field of the Invention
The invention generally pertains to process and apparatus for recording information on a copy sheet by magnetic imaging procedures and is more particularly directed to developing a magnetic latent image with a magnetic toner.
2. Prior Art
The processes for providing latent images on a substrate or surface and then decorating them by a fine pigmented particulate (usually called a toner) to produce a visual image or one that is transferable to a copy sheet are well known in the art.
Generally, in the past, a number of development systems have been used to tone either an electrostatic latent image (zones having electric potential differences between image and non-image areas) or a magnetic latent image (zones having magnetic potential differences between image and non-image areas).
Normally, electrostatic and magnetic toners are not compatible. Electrostatic toners typically do not exhibit marked attraction to magnetic field forces because they are not ferromagnetic while magnetic toners are usually heavy and fairly conductive and are therefore not favored electrostatic charge carriers.
However, some ferromagnetic materials have been used in electrostatic development systems as carriers. These ferromagnetic carrier particles which are relatively large exhibit triboelectric attractions for smaller toner particles and are useful in transferring the toner to an electrostatic image. The toner particles are separated from the carrier by the stronger electrostatic forces on the latent images than the triboelectric forces between carrier and toner.
One example of a cascade development system employing ferromagnetic carrier to transfer a toner to an electrostatic latent image is U.S. Pat. No. 3,545,968 issued to Sato.
Another example of a development apparatus using a ferromagnetic carrier is U.S. Pat. No. 3,437,074 issued to Hagopian et.al. This reference describes a "magnetic brush" development system where the ferromagnetic carrier particles are formed into streamers or bristles and form a brush like mass.
A donor belt utilizing ferromagnetic carrier for toner tranfer in an electrostatic apparatus is disclosed in a U.S. Pat. No. 3,741,790 issued to Wu.
Such development systems rely on the electrostatic forces generated by potential differences in image areas to be stronger than the forces holding the toner particles to the carriers. The electrostatic forces generated by an electrostatic latent image are in fact much stronger than those which can be produced from a magnetic latent image and thus other methods had to be initiated to tone these magnetically.
This has led to the development of using ferromagnetic particles in some form that are not just carriers but actual toners for developing a magnetic latent image. As in the electrostatic development area, there have been a plurality of methods proposed for the decoration of latent magnetic images by magnetic toner.
A cascade development system for magnetic images is illustrated in U.S. Pat. No. 3,250,636 issued to Wilferth. In this reference, magnetic particulate is poured or flooded over a surface containing a magnetic latent image. The toner adheres to the image areas and excess toner flows by gravity from the surface into a reservoir.
Another magnetic toner development system, in which the toner is caused to impinge on a magnetic latent image by flicking the toner from the bristle ends of a wire brush, is illustrated in U.S. Pat. No. 3,825,936 issued to Ott et.al.
Immersion techniques are also known in the art where a tape has a recorded image thereon immersed in a reservoir within a volatile fluid medium. Upon circulation of the fluid medium around the image, toner is attracted to magnetized areas of the image. An example disclosing such a technique is found in U.S. Pat. No. 3,740,265 issued to Springer.
All the aforementioned latent magnetic imaging development apparatus have the problem of contacting toner not only within imaged areas but also within non-imaged areas and thereby producing substantial background. (Toner adhered to non-image areas.)
This is detrimental to a magnetic imaging process as the forces holding the magnetizable particles to the image areas are not as great as those found in electrostatic system and hence background is more difficult to clean from an area after the toner deposition thereon.
A non-contact magnetic imaging system is illustrated in U.S. Pat. No. 3,849,161 issued to Klaenhammer. The system provides alternate magnetizations for image areas in relation to non-image areas. However, such a system is devoid of a process to produce the resolution needed by modern imaging applications in the commercial sector.
Therefore, it would be advantageous to have a clean development system that would also be capable of uniform image development and high resolution.